<?php

/**
 * Class for converting between different unit-lengths as specified by
 * CSS.
 */
class HTMLPurifier_UnitConverter
{
	
	/**
	 * Minimum bcmath precision for output.
	 */
	var $outputPrecision;
	
	/**
	 * Bcmath precision for internal calculations.
	 */
	var $internalPrecision;
	
	/**
	 * Whether or not BCMath is available
	 */
	var $bcmath;
	
	function HTMLPurifier_UnitConverter($output_precision = 4, $internal_precision = 10, $force_no_bcmath = false) {
		$this->outputPrecision = $output_precision;
		$this->internalPrecision = $internal_precision;
		$this->bcmath = !$force_no_bcmath && function_exists('bcmul');
	}
	
	/**
	 * Converts a length object of one unit into another unit.
	 * @param HTMLPurifier_Length $length
	 *	  Instance of HTMLPurifier_Length to convert. You must validate()
	 *	  it before passing it here!
	 * @param string $to_unit
	 *	  Unit to convert to.
	 * @note
	 *	  About precision: This conversion function pays very special
	 *	  attention to the incoming precision of values and attempts
	 *	  to maintain a number of significant figure. Results are
	 *	  fairly accurate up to nine digits. Some caveats:
	 *		  - If a number is zero-padded as a result of this significant
	 *			figure tracking, the zeroes will be eliminated.
	 *		  - If a number contains less than four sigfigs ($outputPrecision)
	 *			and this causes some decimals to be excluded, those
	 *			decimals will be added on.
	 */
	function convert($length, $to_unit) {
		
		/**
		 * Units information array. Units are grouped into measuring systems
		 * (English, Metric), and are assigned an integer representing
		 * the conversion factor between that unit and the smallest unit in
		 * the system. Numeric indexes are actually magical constants that
		 * encode conversion data from one system to the next, with a O(n^2)
		 * constraint on memory (this is generally not a problem, since
		 * the number of measuring systems is small.)
		 */
		static $units = array(
			1 => array(
				'px' => 3, // This is as per CSS 2.1 and Firefox. Your mileage may vary
				'pt' => 4,
				'pc' => 48,
				'in' => 288,
				2 => array('pt', '0.352777778', 'mm'),
			),
			2 => array(
				'mm' => 1,
				'cm' => 10,
				1 => array('mm', '2.83464567', 'pt'),
			),
		);
		
		if (!$length->isValid()) return false;
		
		$n	= $length->getN();
		$unit = $length->getUnit();
		
		if ($n === '0' || $unit === false) {
			return new HTMLPurifier_Length('0', false);
		}
		
		$state = $dest_state = false;
		foreach ($units as $k => $x) {
			if (isset($x[$unit])) $state = $k;
			if (isset($x[$to_unit])) $dest_state = $k;
		}
		if (!$state || !$dest_state) return false;
		
		// Some calculations about the initial precision of the number;
		// this will be useful when we need to do final rounding.
		$sigfigs = $this->getSigFigs($n);
		if ($sigfigs < $this->outputPrecision) $sigfigs = $this->outputPrecision;
		
		// Cleanup $n for PHP 4.3.9 and 4.3.10. See http://bugs.php.net/bug.php?id=30726
		if (strncmp($n, '-.', 2) === 0) {
			$n = '-0.' . substr($n, 2);
		}
		
		// BCMath's internal precision deals only with decimals. Use
		// our default if the initial number has no decimals, or increase
		// it by how ever many decimals, thus, the number of guard digits
		// will always be greater than or equal to internalPrecision.
		$log = (int) floor(log(abs($n), 10));
		$cp = ($log < 0) ? $this->internalPrecision - $log : $this->internalPrecision; // internal precision
		
		for ($i = 0; $i < 2; $i++) {
			
			// Determine what unit IN THIS SYSTEM we need to convert to
			if ($dest_state === $state) {
				// Simple conversion
				$dest_unit = $to_unit;
			} else {
				// Convert to the smallest unit, pending a system shift
				$dest_unit = $units[$state][$dest_state][0];
			}
			
			// Do the conversion if necessary
			if ($dest_unit !== $unit) {
				$factor = $this->div($units[$state][$unit], $units[$state][$dest_unit], $cp);
				$n = $this->mul($n, $factor, $cp);
				$unit = $dest_unit;
			}
			
			// Output was zero, so bail out early. Shouldn't ever happen.
			if ($n === '') {
				$n = '0';
				$unit = $to_unit;
				break;
			}
			
			// It was a simple conversion, so bail out
			if ($dest_state === $state) {
				break;
			}
			
			if ($i !== 0) {
				// Conversion failed! Apparently, the system we forwarded
				// to didn't have this unit. This should never happen!
				return false;
			}
			
			// Pre-condition: $i == 0
			
			// Perform conversion to next system of units
			$n = $this->mul($n, $units[$state][$dest_state][1], $cp);
			$unit = $units[$state][$dest_state][2];
			$state = $dest_state;
			
			// One more loop around to convert the unit in the new system.
			
		}
		
		// Post-condition: $unit == $to_unit
		if ($unit !== $to_unit) return false;
		
		// Useful for debugging:
		//echo "<pre>n";
		//echo "$n\nsigfigs = $sigfigs\nnew_log = $new_log\nlog = $log\nrp = $rp\n</pre>\n";
		
		$n = $this->round($n, $sigfigs);
		if (strpos($n, '.') !== false) $n = rtrim($n, '0');
		$n = rtrim($n, '.');
		
		return new HTMLPurifier_Length($n, $unit);
	}
	
	/**
	 * Returns the number of significant figures in a string number.
	 * @param string $n Decimal number
	 * @return int number of sigfigs
	 */
	function getSigFigs($n) {
		$n = ltrim($n, '0+-');
		$dp = strpos($n, '.'); // decimal position
		if ($dp === false) {
			$sigfigs = strlen(rtrim($n, '0'));
		} else {
			$sigfigs = strlen(ltrim($n, '0.')); // eliminate extra decimal character
			if ($dp !== 0) $sigfigs--;
		}
		return $sigfigs;
	}
	
	/**
	 * Adds two numbers, using arbitrary precision when available.
	 */
	function add($s1, $s2, $scale) {
		if ($this->bcmath) return bcadd($s1, $s2, $scale);
		else return $this->scale($s1 + $s2, $scale);
	}
	
	/**
	 * Multiples two numbers, using arbitrary precision when available.
	 */
	function mul($s1, $s2, $scale) {
		if ($this->bcmath) return bcmul($s1, $s2, $scale);
		else return $this->scale($s1 * $s2, $scale);
	}
	
	/**
	 * Divides two numbers, using arbitrary precision when available.
	 */
	function div($s1, $s2, $scale) {
		if ($this->bcmath) return bcdiv($s1, $s2, $scale);
		else return $this->scale($s1 / $s2, $scale);
	}
	
	/**
	 * Rounds a number according to the number of sigfigs it should have,
	 * using arbitrary precision when available.
	 */
	function round($n, $sigfigs) {
		$new_log = (int) floor(log(abs($n), 10)); // Number of digits left of decimal - 1
		$rp = $sigfigs - $new_log - 1; // Number of decimal places needed
		$neg = $n < 0 ? '-' : ''; // Negative sign
		if ($this->bcmath) {
			if ($rp >= 0) {
				$n = bcadd($n, $neg . '0.' .  str_repeat('0', $rp) . '5', $rp + 1);
				$n = bcdiv($n, '1', $rp);
			} else {
				// This algorithm partially depends on the standardized
				// form of numbers that comes out of bcmath.
				$n = bcadd($n, $neg . '5' . str_repeat('0', $new_log - $sigfigs), 0);
				$n = substr($n, 0, $sigfigs + strlen($neg)) . str_repeat('0', $new_log - $sigfigs + 1);
			}
			return $n;
		} else {
			return $this->scale(round($n, $sigfigs - $new_log - 1), $rp + 1);
		}
	}
	
	/**
	 * Scales a float to $scale digits right of decimal point, like BCMath.
	 */
	function scale($r, $scale) {
		return sprintf('%.' . $scale . 'f', (float) $r);
	}
	
}
